Deliver mailbox mail as parent inbound so Skywalker stays idle

[TheGreatAxios]

Summary

Occupancy delivers worker done/fail reports as system inbound so an idle Skywalker starts a new turn, including while siblings still run. TUI-primary wait_agents yields as a timeout (workers untouched) when mailbox mail, a parked ask, or a queued Enter steer is ready. Nested orchestrators still collect with wait_agents.

The mailbox already holds terminal reports and director questions. What was wrong was wait_agents as a parent-busy tool: Skywalker sat inside a blocking read, so Enter queued until the wait returned. Idle-with-fleet already lets the parent go idle after spawn; collection is still a poll unless mail wakes the parent.

• Already-collected waits return status without a second report body (no double-deliver).
• Skywalker prompt and docs say spawn then idle; do not poll wait_agents. list_agents stays.
• Fleet-dry occupancy still owns the last-lane 1→0 + open-tasks shot.
• Yield is timeout-shaped; workers are not interrupted.

Verification

• bun run typecheck, bun run build, and bun run test pass
• bun run check passes (lint, typecheck, build, guarded suite, seed 424242)
• Critic: no blocking findings on the TUI happy path

Fixes CL-7518

[linear-code]

CL-7518